Relay via Office 365 stopped working - Client host blocked using Spamhaus

Issue

Relaying email via Office 365 suddenly stopped working. Relayed emails were sent from a Windows 2012 Server using a simple PowerShell script. After logging in to the server and running the PowerShell script in an interactive mode following error was displayed:

Exception calling "Send" with "1" argument(s): "Mailbox unavailable. The server response was: 5.7.1 Service unavailable, Client host [212.159.94.214] blocked using Spamhaus. To request removal from this list see http://www.spamhaus.org/lookup.lasso
(AS16012612) [VE1EUR01FT026.eop-EUR01.prod.protection.outlook.com]"

Hyper-V VM move fails with 0x8009030D error (credentials not recognized)

Issue

Trying to move a virtual machine from one Hyper-V 2016 host to another while running Hyper-V Management Tools from a separate management server. Following error is displayed shortly after starting the move operation:

There was an error during move Operation.
Virtual machine migration Operation failed at migration source.
Failed to establish a connection with host "DESTINATION-SERVER": The credentials supplied to the package were not recognized (0x8009030D).
The Virtual Machine Management Service failed to authenticate the connection for a Virtual Machine migration at the source host: no suitable credentials available. Make sure the Operation is initiated on the source host of the migration, or the source host is configured to use Kerberos for the authentication of migration connections and Constrained Delegation is enabled for the host in Active Directory.

Live migrations are not enabled on this computer (Hyper-V)

Issue

Trying to move a virtual machine between Hyper-V 2016 hosts. Following error is displayed:

Live migrations are not enabled on this computer.
This computer is not configured for live migration of virtual machines. To enable live migration, modify the Hyper-V settings.

Deleting Default Mailbox Database in MS Exchange 2016

If you try to remove the default mailbox database in Exchange 2016 EAC (Exchange Admin Center) you may get following error:

This mailbox database contains one or more mailboxes, mailbox plans, archive mailboxes, public folder mailboxes or arbitration mailboxes, Audit mailboxes, To get a list of almailboxes in this database, run the command Get-Mailbox -Database <Database ID>. To get a list of ail mailbox plans in this database, run the command Get-MailboxPlan...

This is because the default database contains number of system mailboxes that are normally not visible in GUI. For example arbitration, monitoring, AuditLog mailboxes.